Communications networks, such as a public switched telephone network (PSTN), may provide services to a subscriber-end device, such as a telephone or modem, by allowing the subscriber-end device to connect to the communications network. For example, a telephone may connect to the PSTN by entering an active state thereby enabling communication between the telephone and the PSTN. The active state may be referred to as off-hook and the inactive state may be referred to as on-hook.
The subscriber-end device may be required to exhibit certain electrical characteristics when connected to the communications network. Moreover, different communications networks may require different electrical characteristics and exhibit different failure modes. For example, the electrical termination characteristics of some countries may be met using a constant-resistance termination (constant-resistance mode) while other countries, such as France, may be met using a constant-current limiting termination (constant-current mode). Many subscriber-end devices, therefore, use a programmable network interface circuit (NIC) to provide the electrical characteristics for the particular communications network to which the subscriber-end device is connected.
FIG. 1 graphically illustrates the electrical characteristics of two exemplary communications networks. The line 101 shows the electrical characteristics, referred to as constant-resistance mode, of a telephone connected to a PSTN wherein the current is controlled by the NIC to present a constant resistive load to the PSTN in relation to the voltage provided by the communications network. The constant-resistance mode may present a low-resistance load of about 100 ohms or high-resistance load of about 300 ohms. The line 102 shows the electrical characteristics, referred to as constant-current mode, of a telephone connected to a PSTN wherein the current is kept constant over a range of voltages provided by the communications network.
The communications networks described above may not perform as specified. For example, a PSTN operating in constant-resistance mode may be specified to source a current not to exceed 120 milliamperes (ma) as shown by the dotted line in FIG. 1. If, however, a communications network failure occurs, the source current may exceed 120 ma, causing excessive power dissipation and thereby possibly damaging the NIC. Similarly, a PSTN operating in constant current mode may be specified to provide a network voltage not to exceed 70 volts. If, however, the voltage provided by the communications network exceeds the specified maximum voltage, the NIC may be damaged due to excess power dissipation.
Alternatively, a user may inadvertently connect the NIC to a communications network which is incompatible with the electrical characteristics described herein. For example, Private Branch Exchange (PBX) networks may provide a relatively large current to connected subscriber-end devices. The physical connection to the PBX, however, is such that a subscriber-end device intended for a PSTN may be mistakenly connected to the PBX. Consequently, the current provided by the PBX may cause excessive power dissipation in the NIC, thereby possibly damaging the NIC.
As illustrated in FIG. 2, existing systems may use an in-line device 202 to protect the NIC 201 from the damage described above. The in-line device 202 may be a fuse or resettable device (such as a relay) which de-couples the communications network 203 from the NIC 201 before NIC 201 is damaged. Fuses may need to be replaced once the failure occurs. Resettable devices may need to be reset once the failure occurs. Fuses and resettable devices may also be packaged in such a way so as to be undesirable for some applications requiring relatively high levels of integration. For example, a fuse or relay may be undesirable for use in an integrated modem of a laptop computer due to space and power restrictions. Moreover, a fuse or resettable device may not readily provide an indication that a failure has or is about to occur. For example, if the PSTN sources too much current, the user may be unaware that the NIC may soon fail unless the NIC is de-coupled from the communications network. In view of the above, there is a need for an improved NIC protection.